1. Field of the Invention
The present invention relates to a variable valve system which can change a maximum lift amount of at least either of an inlet valve and an exhaust valve which are engine valves in an internal combustion engine.
2. Description of Related Art
There has been known a variable valve system for an internal combustion engine which includes a transmission mechanism for transmitting a valve actuating force of a valve cam to an engine valve and changing a maximum lift amount of the engine valve by being actuated by a control member and a biasing member for generating a biasing force which brings a rocker member which makes up the transmission mechanism into contact with the valve cam (for example, refer to Japanese Patent Unexamined Publication JP-A-2005-315182). In the variable valve system, the rocker member is supported on a holder, which is driven to be displaced by the control member, in such a manner as to oscillate thereon, and the biasing member applies biasing force to an acting portion of the rocker member so as to bring a cam contact portion of the rocker member into contact with the valve cam.
[First Problem]
In the rocker member which makes up the transmission mechanism of the variable valve system, in the event that a cam contact point of the cam contact portion with the valve cam is spaced away from a biased contact point of the acting portion with the biasing member due to, for example, the cam contact portion and the acting portion being provided in opposite positions across an oscillation center line, this causes not only the enlargement in size but also the reduction in rigidity of the rocker member. Then, when the rigidity of the rocker member is reduced, since the rocker member is made easy to be deformed by virtue of the biasing force of the biasing member, the following capability of the rocker member to the valve cam is reduced, whereby the opening and closing accuracy of the engine valve is reduced. Then, to cope with this, when attempting to increase the rigidity of the rocker member by increasing the thickness thereof, the rocker member has to be enlarged, and the weight thereof also has to be increased, thereby an enlargement in size and an increase in weight of the transmission mechanism having to be called for. In addition, when attempting to increase the biasing force of the biasing member so as to prevent the reduction in following capability of the rocker member which is attributed to an increase in inertial mass thereof, power is increased which is necessary to actuate the valve cam against the biasing force applied to the valve cam via the rocker member, this resulting in deterioration in fuel economy.
Further, there have been known a variable valve system for an internal combustion engine which includes a transmission mechanism for transmitting a valve actuating force of a valve cam to an engine valve and changing a maximum lift amount of the engine valve by being actuated by a control member and a biasing member for generating a biasing force which brings the transmission mechanism into contact with the valve cam, wherein the transmission mechanism is made up of a sub-cam having a cam contact portion which contacts the valve cam, a rocker arm which pressurizes the engine valve and which is driven by the sub-cam and a holder which supports the sub-cam and the rocker arm in such a manner as to oscillate thereon (refer to the JP-A-2005-315182).
[Second Problem]
In a sub-cam which makes up a transmission mechanism of a variable valve system, in the event that a fulcrum portion which is supported on a holder in such a manner as to oscillate thereon is disposed between a cam contact portion with which a valve cam is brought into contact and an acting portion with which a biasing member is brought into contact, a distance extending from the acting portion to an input portion across the fulcrum portion becomes long, and because of this, the sub-cam is enlarged in size and the rigidity of the sub-cam is reduced. In addition, in the event that the rigidity of the sub-cam is reduced, since the sub-cam tends to be easily deformed by virtue of the biasing force of the biasing member, the following capability of the sub-cam to the valve cam is reduced, whereby the opening and closing control accuracy of the engine valve is reduced. Then, when attempting to enhance the rigidity of the sub-cam by increasing the thickness thereof, the sub-cam is enlarged in size, and this calls for increase in size and weight of the transmission mechanism. In addition, when attempting to increase the biasing force of the biasing member in order to prevent the reduction in following capability which is attributed to an increase in inertial mass of the sub-cam, power is increased which is necessary to drive the valve cam against the biasing force which is applied to the valve cam through the sub-cam, and the fuel economy is deteriorated.
Furthermore, in the event that the biasing member is held on to the holder, when the biasing member protrudes from the transmission mechanism, the variable valve system is made larger in size by such an extent that the biasing member protrudes from the transmission mechanism.
Furthermore, there has been known a variable valve system for an internal combustion engine which includes a transmission mechanism for transmitting a valve actuating force of a valve cam to an engine valve and changing a maximum lift amount of the engine valve by being driven by a control member, wherein the transmission mechanism is made up of a sub-cam which contacts the valve cam, a rocker arm which pressurizes the engine valve and which is driven by the sub-cam and a holder which supports the sub-cam and the rocker arm in such a manner as to oscillate thereon and which is driven by the control member (refer to the JP-A-2005-315182).
[Third Problem]
In the event that a holder which makes up a transmission mechanism of a variable valve mechanism supports a rocker arm in such a manner as to oscillate thereon and a control force from a control member is applied to the holder, since the control force is applied to the holder through the rocker arm in addition to a valve actuating force from a valve cam and a reaction force from an engine valve, in order to enhance the control accuracy of opening and closing control of the engine valve which includes control of maximum lift amount, the rigidity of the holder needs to be enhanced so as to suppress the generation of deformation of the holder due to the loads. However, attempting to enhance the rigidity of the holder calls for an increase in size and weight of the holder.